Superconducting quantum interference devices (SQUID) have been commercially available for several years. SQUIDs are the most sensitive magnetic field or small voltage sensors currently available. The operation of SQUID sensors is based on two effects which can be observed only in the presence of superconductivity. These are flux quantization and Josephson effects. SQUID sensors generally use one or two Josephson junctions connected in a closed superconducting loop.
SQUID systems have taken on a number of different forms but what has become an accepted form for thin film implementations is the "washer" design which achieves low inductance in the SQUID loop and tight coupling to multi-turn input coils by making the loop into a slotted groundplane. This design resulted in the first practical thin film SQUID to be realized in a planar geometry. Very sensitive, low-noise devices with usefully large input coil inductance have been fabricated over the years using this design. This widely used washer design is described in Jaycox et al., Planar Coupling Scheme For Ultra Low Noise DC SQUIDs, IEEE Trans. Magn, MAG-17, p. 400-403 (January 1981).
A modulation coil of this traditional design comprises a single turn loop around the outside of the multi-turn signal coil. This results in high mutual inductance between the modulation and input coils, which is undesirable in a practical system because drive currents injected into the modulation coil will appear as an output from the signal coil. This is analogous to the problem created by using an unbalanced mixer in radio receiver circuitry. An example of a modulation coil used in conjunction with a SQUID is shown in EP-A-O 337 123.
For many applications, it is not desired that the SQUID loop itself be sensitive to uniform magnetic fields because magnetic flux should only be coupled into it through the signal coil. It is possible to fabricate double washer designs in which the two washers are configured as a gradiometer to reject the effects of uniform fields. In these designs, however, the bias current which must pass through the Josephson junctions becomes magnetically coupled into the SQUID loop. This results in an undesirable interaction which can introduce noise and drift into the SQUID sensor from the drive electronics.
The non-symmetrical way in which bias currents are introduced into the junctions also makes the SQUID unduly sensitive to common mode noise which may be picked up on the bias leads which run from the electronic drive package at room temperature down to the SQUID sensor in the cryogenic environment. Again, this noise becomes an influence on the output signal.
A reference which suggests constructing parallel SQUID loops is U.S. Pat. No. -A-4 064 029. Elimination of crosstalk between coils of a SQUID is described in Proceedings of the IEEE, vol. 77, No. 8, Aug. 1989, pages 1208-1223. None of the mentioned-references suggest the invention described herein. coil. This makes the device more unilateral which is a desirable feature. Further, currents flowing into the Josephson junctions are not coupled to the SQUID loop. This makes the device more insensitive to fluctuations or noise in the bias current circuitry. Additionally, common mode noise on the bias leads, modulation coils or signal coils does not couple into the junctions.
These improvements are accomplished by employing a balanced thin film dc SQUID system comprising a substrate, a superconductive groundplane layer on said substrate, said superconductive groundplane layer being formed with a slit having an enlarged opening at each end thereof, thin film Josephson junction means located on said substrate, said Josephson junction means being part of a SQUID loop, means for interconnecting said Josephson junction means, a thin film signal coil in coupling relationship with each said enlarged opening at opposite ends of said slit, a modulation coil in coupling relationship with said signal coil, and means for applying bias current to said Josephson junction means, characterized in that: said superconductive groundplane layer is formed with first and second slits forming a mutual intersection intermediate their ends, each said slit having an enlarged opening at each end thereof; said Josephson junction means being located on diagonally opposite corners of said groundplane at said intersection of said slits; said means for interconnecting said Josephson junction means extending across said intersection; and an electrically balanced, physically symmetrical pair of thin film modulation coils, each said modulation coil being in coupling relationship with one of said enlarged openings at opposite ends of the other of said first and second slits; said bias current being applied to said Josephson junction means in a way so as to not disturb the symmetry created by said modulation coil arrangement.
Other aspects of the symmetry of the system of the invention will be described. Four separate electrical leads are provided by which the Josephson junctions are biased. Two of the leads are connected to one side of the parallel Josephson junctions, while the other two leads are connected to the large groundplane structure of the SQUID loop. The external circuit is designed to preserve the highly balanced nature of the chip and force equal magnitudes of current to flow in all four of the bias leads. An additional feature is a superconducting shield layer on top of the SQUID chip which further reduces leakage inductance and RFI sensitivity, and improves coupling in the signal and modulation coils.